The role of the small GTP-binding proteins as signaling molecules has been studied extensively in many cell types, but their importance has been somewhat neglected in the field of neuroscience. Until recently members of the Rho subfamily of GTPases were believed to be involved primarily in the regulation of cytoskeletal organization in response to extracellular growth factors. However, research from a number of laboratories has revealed that small GTPases play a crucial role in diverse cellular events such as membrane trafficking, transcriptional regulation and cell growth control and development. The signaling pathways that couple membrane receptors to GTPases, GTPases to actin cytoskeleton, and GTPases to additional effector molecules are beginning to be discovered, especially in neurons. One member of the Rho GTPase subfamily, Rac, has been implicated in regulation of the mitogenic response, superoxide generation, regulation of transcription, morphological changes necessary for migration, activation of the release sites for exocytosis and specific cytoskeleton rearrangements that play a crucial role in cell motility and cytokinesis. Regarding the nervous system, neuronal development involves several discrete morphological steps requiring migration of newborn neurons to characteristic locations, extension of axons and dendrites into proper target regions, and formation of synapses with appropriate partners. The small GTPases, including Rac, are believed to be critical regulators of these processes. It has been reported that Rac participates in neuronal morphogenesis including migration, polarity, axon growth and guidance, dendrite elaboration, plasticity and synapse formation. We have found that Rac is highly expressed in mouse hippocampus where it is equally distributed between cytosolic and membrane fractions, indicating that this protein might be cycling between inactive and active forms. In addition, our preliminary data shows that NMDA receptor activation in hippocampal slices causes Rac to translocate to the membrane in a manner similar to that observed in phagocytic cells. It is likely that Rac is an important molecule involved in synaptic plasticity, playing a role in the morphological changes and activation of signal transduction pathways associated with hippocampal learning and memory. I propose in this application to investigate whether Rac plays an active role in LTP and hippocampus dependent memory formation by addressing the following Specific Aims: 1) to test the hypothesis that NMDA receptor activation results in membrane translocation and activation of Rac in hippocampal area CA1, 2) to test the hypothesis that LTP-inducing stimulation is associated with membrane translocation and activation of Rac and that Rac activation is necessary for LTP and 3) to test the hypothesis that Rac membrane translocation and activation is both associated with and necessary for hippocampus-dependent memory formation. The results of these experiments will provide important information regarding the signaling mechanisms that underlie synaptic plasticity, as well as learning and memory processes, which in turn will provide insights into the basis of diseases involving memory impairment, such as non-syndromic X-linked mental retardation (MRX), Fragile X mental retardation, Alzheimer's disease, William's syndrome, Angelman syndrome (AS), forebrain ischemia, and schizophrenia.